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Author Topic:   Water Pressure Gauge Readings

bigjohn1 posted 06-20-2005 08:16 AM ET (US)
I finally got around to hooking up the water pressure gauge to my 2004 Merc 115-EFI and am wondering how to interpret the readings. My factory manual lists a very wide tolerance value for this readings, 5 PSI to 30 PSI at 5,000 RPM as I recall, and, I suppose it goes without saying, as long as you're in this range, pressure is OK by Mercury standards. I am more interested in properly interpreting the pressure reading as it applies to ventilation of the prop. In other words, if I am showing a constant reading of, say 20 PSI at 3,000 RPM and then my prop begins to ventilate slightly due to too much trim, how will the gauge react? Will the pressure fall off only slightly or will I realize a significant drop? My real goal I suppose is to use the gauge as a supplement to my hearing as I experiment with various engine heights and performance props in the coming weeks. Is the gauge a reliable indicator of propeller blade ventilation?
bsmotril posted 06-20-2005 08:56 AM ET (US)
I have some experience running a flats boat with a Yamaha 150 TRP on a jackplate, equipped with a water pressure gauge. Idle water pressure is around 3-5 psi, about 12 psi at slow cruise, and 18-20 psi at WOT. When you raise the jackplate, the prop will start to ventilate before you loose water pressure. BUT, this is on a tunnel hull, and your boat will likely behave differently. We'll raise the plate until RPMs start to rise, then drop it down about an inch. When run at this trim setting, the motor is very succepbtible to disrupted water flow due to grass or seaweed. Anything wrapping around the front of the motor disrupts the water flow, and the pressure will drop from 20 psi to 0 in a split second as the laminar flow around the gearcase is disrupted from the grass, and the motor starts sucking air into the cooling system. The prop won't ventilate in these circumstances, even though you loose water pressure. A close eye on the pressure gauge is needed when there's a lot of floating dead grass.
I don't think you can predict the behavior or relationship of prop blow out to water pressure unless you try a particular height, prop, hull combination and find out for yourself. There are just too many variables, like prop cup, and hull shape, that will dictate how a particular combination will behave. BillS
bigjohn1 posted 06-20-2005 06:42 PM ET (US)
Thanks Bill, I appreciate the feedback.
Sal DiMercurio posted 06-20-2005 08:40 PM ET (US)
Prop blowout wont indicate anything on your water pressure guage, especially on a merc where the pickup is in front of the prop.
All engines read different water pressure, no two engines will be the same because one might have a brand new empellor & the other might have a 3 year old one.
For instance, my 200 hp Evinrude FICHT runs about 25lbs at 3000 rpms & my 1991 Johnson 150 ran at about 18 lbs at 3000 rpms.
It's a good idea to check your water pressure with a new water pump installed & remember it, this way when you notice a pressure drop at your normal cruise rpm you know your pump is getting weak.
As far as relying on a water pressure guage to see if your prop blew out, I would just hear it blow out & feel the boat slowing down instead.
Sal
Chuck Tribolet posted 06-20-2005 09:32 PM ET (US)
I see two different failure modes.
One is that the impeller gets old and tired. An occasional
glance at a pressure guage will detect that.
The other is that something sudden happened. Maybe you
wrapped kelp or a plastic bag around the intakes. Maybe
bsmotril's distrupted flow happened. ... The analog
guage is unlikely to notice that, and a simple low pressure
alarm will be obnoxious at startup. It would be nice to have
an electronic widget that will notice that the engine is running AND the pressure is low.

Chuck
jimh posted 06-20-2005 10:20 PM ET (US)
I suppose there might be a correlation between the internal pressure of the water cooling system and the propeller blade ventilating from air drawn in from the surface, but I don't know that the typical gauge would be sensitive enough to reliably show it.
What I see in the pressure gauge readings is a reaction to the cooling system's thermostat and thermostat bypass valve positions. Generally the water pressure tends to increase with engine speed, but at some point on my motor--it may not be applicable with your motor--a cooling system valve opens and the pressure drops.
The pickup tube for the water pressure gauge is located almost at the aspirator, and upstream of both of them is a bypass valve that allows more water to flow into the cooling system. When this valve opens, less water gets to the pressure gauge pick-off point and to the aspirator. (The volume of water coming out of the aspirator decreases, too.) The pressure gauge drops a bit when this happens. The extra water gets diverted into the cooling jacket, I think.
If nothing else, the readings you are getting now will be a bench mark for later. If your engine water pump begins to loose pressure or act differently, you will have something to compare it with.
an86carrera posted 06-21-2005 11:45 AM ET (US)
I have chosen use my pitot sp? tube type speedo for my water pressure indicator. I found this guage useless with a GPS on the boat.
But now I have a very large centrally located sensitive gauge. My Ficht 200 runs from 10 mph to 45mph at WOT. Although I no longer have a psi reading as a reference tool I find this very to be so obviously in my face that I look at it alot and can quickly notice any blockage in the system.
Also, it seems very sensitive and in the year it's been like this I am watching my water pump wear out on the guage... originally I had 47mph at WOT.
bsmotril posted 06-21-2005 11:58 AM ET (US)
Now that is one creative idea. I'll be buying tubing and a T connector this afternoon. BillS
jimh posted 06-21-2005 12:22 PM ET (US)
The notion of using the speedometer gauge as the water pressure gauge also occured to me. It was more for the concept of "what can I put in that big 3-3/8-inch hole," but I do think the larger gauge would have more resolution.
The next step would be to disassemble the speedometer and recalibrate the dial face.

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Author	Topic: Water Pressure Gauge Readings
bigjohn1	posted 06-20-2005 08:16 AM ET (US) I finally got around to hooking up the water pressure gauge to my 2004 Merc 115-EFI and am wondering how to interpret the readings. My factory manual lists a very wide tolerance value for this readings, 5 PSI to 30 PSI at 5,000 RPM as I recall, and, I suppose it goes without saying, as long as you're in this range, pressure is OK by Mercury standards. I am more interested in properly interpreting the pressure reading as it applies to ventilation of the prop. In other words, if I am showing a constant reading of, say 20 PSI at 3,000 RPM and then my prop begins to ventilate slightly due to too much trim, how will the gauge react? Will the pressure fall off only slightly or will I realize a significant drop? My real goal I suppose is to use the gauge as a supplement to my hearing as I experiment with various engine heights and performance props in the coming weeks. Is the gauge a reliable indicator of propeller blade ventilation?
bsmotril	posted 06-20-2005 08:56 AM ET (US) I have some experience running a flats boat with a Yamaha 150 TRP on a jackplate, equipped with a water pressure gauge. Idle water pressure is around 3-5 psi, about 12 psi at slow cruise, and 18-20 psi at WOT. When you raise the jackplate, the prop will start to ventilate before you loose water pressure. BUT, this is on a tunnel hull, and your boat will likely behave differently. We'll raise the plate until RPMs start to rise, then drop it down about an inch. When run at this trim setting, the motor is very succepbtible to disrupted water flow due to grass or seaweed. Anything wrapping around the front of the motor disrupts the water flow, and the pressure will drop from 20 psi to 0 in a split second as the laminar flow around the gearcase is disrupted from the grass, and the motor starts sucking air into the cooling system. The prop won't ventilate in these circumstances, even though you loose water pressure. A close eye on the pressure gauge is needed when there's a lot of floating dead grass. I don't think you can predict the behavior or relationship of prop blow out to water pressure unless you try a particular height, prop, hull combination and find out for yourself. There are just too many variables, like prop cup, and hull shape, that will dictate how a particular combination will behave. BillS
bigjohn1	posted 06-20-2005 06:42 PM ET (US) Thanks Bill, I appreciate the feedback.
Sal DiMercurio	posted 06-20-2005 08:40 PM ET (US) Prop blowout wont indicate anything on your water pressure guage, especially on a merc where the pickup is in front of the prop. All engines read different water pressure, no two engines will be the same because one might have a brand new empellor & the other might have a 3 year old one. For instance, my 200 hp Evinrude FICHT runs about 25lbs at 3000 rpms & my 1991 Johnson 150 ran at about 18 lbs at 3000 rpms. It's a good idea to check your water pressure with a new water pump installed & remember it, this way when you notice a pressure drop at your normal cruise rpm you know your pump is getting weak. As far as relying on a water pressure guage to see if your prop blew out, I would just hear it blow out & feel the boat slowing down instead. Sal
Chuck Tribolet	posted 06-20-2005 09:32 PM ET (US) I see two different failure modes. One is that the impeller gets old and tired. An occasional glance at a pressure guage will detect that. The other is that something sudden happened. Maybe you wrapped kelp or a plastic bag around the intakes. Maybe bsmotril's distrupted flow happened. ... The analog guage is unlikely to notice that, and a simple low pressure alarm will be obnoxious at startup. It would be nice to have an electronic widget that will notice that the engine is running AND the pressure is low. Chuck
jimh	posted 06-20-2005 10:20 PM ET (US) I suppose there might be a correlation between the internal pressure of the water cooling system and the propeller blade ventilating from air drawn in from the surface, but I don't know that the typical gauge would be sensitive enough to reliably show it. What I see in the pressure gauge readings is a reaction to the cooling system's thermostat and thermostat bypass valve positions. Generally the water pressure tends to increase with engine speed, but at some point on my motor--it may not be applicable with your motor--a cooling system valve opens and the pressure drops. The pickup tube for the water pressure gauge is located almost at the aspirator, and upstream of both of them is a bypass valve that allows more water to flow into the cooling system. When this valve opens, less water gets to the pressure gauge pick-off point and to the aspirator. (The volume of water coming out of the aspirator decreases, too.) The pressure gauge drops a bit when this happens. The extra water gets diverted into the cooling jacket, I think. If nothing else, the readings you are getting now will be a bench mark for later. If your engine water pump begins to loose pressure or act differently, you will have something to compare it with.
an86carrera	posted 06-21-2005 11:45 AM ET (US) I have chosen use my pitot sp? tube type speedo for my water pressure indicator. I found this guage useless with a GPS on the boat. But now I have a very large centrally located sensitive gauge. My Ficht 200 runs from 10 mph to 45mph at WOT. Although I no longer have a psi reading as a reference tool I find this very to be so obviously in my face that I look at it alot and can quickly notice any blockage in the system. Also, it seems very sensitive and in the year it's been like this I am watching my water pump wear out on the guage... originally I had 47mph at WOT.
bsmotril	posted 06-21-2005 11:58 AM ET (US) Now that is one creative idea. I'll be buying tubing and a T connector this afternoon. BillS
jimh	posted 06-21-2005 12:22 PM ET (US) The notion of using the speedometer gauge as the water pressure gauge also occured to me. It was more for the concept of "what can I put in that big 3-3/8-inch hole," but I do think the larger gauge would have more resolution. The next step would be to disassemble the speedometer and recalibrate the dial face.